This invention relates to methods of making telecommunications cable.
In telecommunications cable designs having twisted pairs of conductors in their cores, it is a design ideal to provide inductive and capacitive balance between the conductors of each pair. Such balance depends upon maintaining the conductors a specific distance apart along their twisted lengths, but unfortunately in practice, specific distance maintenance is impossible to achieve. While insulation thickness on a conductor is controllable within extremely narrow limits, absolute control upon the twisting operation so as to achieve a constant distance between the conductors is impossible as pressure between the surfaces of the conductor insulation is known to vary and localized gaps may even be present between these surfaces along the twisted pair lengths. This manufacturing problem provides the main reason for capacitive and inductive imbalances along the cable and, in this respect, tends to increase cross-talk between conductor pairs in cable. Cross-talk becomes a more exaggerated and serious problem for cables when transmitting high frequency signals. Solutions to the above problem have long been desirable but without positive results.
In addition, there may be tightening or loosening of the twist between conductors of a pair when the pair is being formed, together with other pairs, into a cable core. This tightening or loosening effect only serves to exacerbate the capacitance and inductance problems.
Suggestions have been made, for instance, as in U.S. Pat. No. 3,102,160, for insulating both conductors of a pair simultaneously with a single insulation which extends between and encompasses both conductors. However, while such a common insulation is extruded onto the conductors to maintain the distance apart of the conductors at any position along their lengths, it does not solve the capacitance or inductance problems. This is because the distance apart of the conductors is not controllable as they are fed through the extrusion orifice while being coated with the insulating material. Perhaps one of the reasons for this is that because of the shape of the extrusion orifice, the extrusion forces are not balanced and there is a lateral pull upon the conductors which needs to be corrected.
A further problem which applies to twisted conductors having their own individual insulation, is that while efforts have been made as mentioned above to maintain the distances between the conductors constant along their lengths, it has been found that upon incorporation into cable cores and also during bending of cable in use, that at certain positions conductors of certain pairs are likely to move apart under the different tensions and bend radii which are applied and thus the capacitance and inductance imbalance then becomes more exaggerated.